For many years, those skilled in the design of hydraulic systems have recognized that under some circumstances operating cylinders or motors must be protected from over pressurization when the control spool of the associated control valve is in a blocked position which prevents flow from the operating cylinders. Often, a relief valve has been connected to the cylinder port of the control valve to prevent such overpressurization. Frequently, such a relief valve has been incorporated into the control valve itself to provide a more compact and economical package.
In some applications, there are also conditions where mechanical linkages may interact to create overpressurization of one operating cylinder when another operating cylinder is functioning. In these situations, the usable hydraulic force for the other operating cylinder may be reduced and stalling of the system may be experienced due to reduced flow. To overcome this type of difficulty, prior systems have included separate overload relief valves, sequence valves and control valves, all plumbed into such hydraulic systems to reduce or unload the pressure acting at the cylinder port of the one operating cylinder.
One example of prior art hydraulic systems in which mechanical linkages may interact to cause overpressurizing of a motor and stalling of a pump is disclosed in U.S. Pat. No. 3,410,427 granted to McCarthy for a refuse packing system. A refuse truck is provided with a ram or packer panel driven by an hydraulic motor which forces refuse into a storage compartment where it is compressed against an ejector panel driven by another hydraulic motor. As packing of the refuse proceeds, the pressure in the hydraulic motor for the ejector panel rises as the refuse is compacted. Eventually, it becomes necessary to relieve the pressure in the hydraulic motor for the ejector panel so that the panel can move into the refuse storage compartment, thereby allowing the packer panel to force more refuse into the storage compartment and preventing stalling of the system. In such prior art systems, relief of excess pump discharge pressure and excess pressure in the hydraulic motor for the ejector panel has been achieved by plumbing into the hydraulic system, separately from the control valve, a first valve for relieving system pressure at a first predetermined pressure and a second valve for relieving the pressure in the hydraulic motor to the ejector panel at a second, predetermined pressure. In one actual prior art system embodied in a refuse truck, the sensing line for relief of pump discharge pressure was connected upstream of the control valve for the ejector panel motor which caused the valve to release very quickly and resulted in poor compaction. A need has continued to exist for a control valve incorporating additional relief valves responsive to pump discharge pressure, cylinder pressure, or both.